Hi @Alex,
not sure what you mean by heterogeneous K values. I suspect you mean material properties like permeability or thermal conductivity.
Dependent on your type of heterogeneity, you could use a Function parameter or set a MeshElement parameter that is defined in your bulk mesh. This was discussed here: Question: The setting of heterogeneous parameters in OGS - #2 by thomas.fischer. To add cell or point field to your mesh file, you could use paraview, or python tools like meshio or VTUinterface. This was also discussed on the discourse earlier.